Ultrasound transducers and transducer arrays are widely used in both medical imaging and imaging for nondestructive testing (NDT) and/or nondestructive evaluation (NDE). However, the manufacture of one dimensional and two dimensional ultrasound transducer arrays presents a number of challenges.
Conventional dicing based fabrication of large numbers of small array elements is a slow and expensive process, especially for PMN-PT single crystal array fabrication. Furthermore, high electrical impedance per element in 2-D arrays results in a poor match to transmitter/receiver (T/R) electronics and poor signal to noise ratios (SNR).
Three methods have been used to try to develop 2-D transducer array technology: 1) Use of multilayer ceramic processing (co-firing) or composite stacking to form low impedance, high sensitivity array elements; 2) conventional array fabrication (dicing) with hybrid electronic interconnect methods to integrate preamplifier circuitry into the transducer head; and 3) capacitive micromachined ultrasound transducer (cMUT) technology.
The first of these methods makes use of ceramic tape casting and screen printing (multilayer ceramic capacitor and ceramic electronic substrate manufacturing methods) to form arrays of multilayer piezoelectric resonators with higher capacitance than single diced elements. This method is very good at decreasing element impedance since it varies as the inverse square of the layer count. However, significant difficulties are associated with interconnecting the layers in each element to the array cable which requires vias running to each element. The minimum size and spacing of via holes that can be formed by ceramic packaging methods are on the order of the array element spacing. Therefore, the inter-element spaces would be largely filled with metal conductor vias, which would result in unacceptable levels of inter-element cross talk. Stacking diced composites and composites made by injection molding have also been attempted. However, in these cases no acceptable interconnection method has been developed.
The use of hybrid electronic interconnections to integrate preamplifier circuitry into the transducer head has been successfully commercialized. This greatly reduces the SNR problem associated with high impedance elements by eliminating the coaxial cable connection between transducer and preamplifier. However, because it is a diced array with a very large number of elements, it is very expensive and therefore practically limited to premium systems.
cMUTs may provide significant contributions to piezoelectric transducer technology, but they also represent a great challenge to manufacturing technology. cMUTs are an application of microelectromechanical systems (MEMS) technology. cMUTs consist of diaphragms of silicon nitride suspended over a tiny cavity. Electrostatic forces are used to vibrate the membrane producing an ultrasonic wave. In most applications, cMUTs are made using photolithography using a batch wafer processes, providing a potentially a low cost, 2-D array technology. cMUTs also have a very good acoustic impedance match to tissue, and the effective coupling coefficient can approach 1 for some modes of operation. These two properties combine to give cMUTs a very broad band response. It also has the potential of being easily integrated to T/R electronics. Currently, most cMUTs are made using silicon on insulator (SOI) which requires hybrid techniques for integration. Despite the great promise of this technology there are several drawbacks.
First, cMUTs suffer from reduced sensitivity compared to piezoelectric transducers. This is due mainly to parasitic capacitance. Second, cMUTs require a DC bias to operate. This can complicate system design particularly for catheter and endoscope transducers. cMUT reliability is uncertain due to reliance on a bending mode resonance. cMUT elements are also prone to failure due to stiction. Furthermore, it is very difficult to apply cMUT technology to high frequency transducer arrays.
A variation of cMUT technology uses piezoelectric thin films to drive the diaphragms. The main advantage is that no bias is needed, but the bandwidth and sensitivity is very poor due to the low coupling resonance mode and the variable properties of thin film piezoelectric.
These and other drawbacks are found in current transducer array systems.
What is needed are one dimensional (1-D) and two dimensional (2-D) arrays and a method for making them that is cost effective while providing a consistently reliable product from frequencies less than 2 MHz to greater than 100 MHz.